The ultrasonic transducer is used in a device for diagnosing a tumor in a human body by transmitting and receiving ultrasonic waves.
The ultrasonic transducer utilizing the vibration of a piezoelectric body has been used so far. However, with the recent progress in the MEMS technology, a capacitive micromachined ultrasonic transducer (CMUT) in which a vibrating portion having a structure in which a cavity portion (gap) is sandwiched between electrodes is fabricated on a silicon substrate has been actively developed for achieving its practical use.
For example, U.S. Pat. No. 5,894,452 (Patent Document 1) discloses the CMUT cell in which a cavity portion is formed by etching an insulator sandwiched between electrodes. In this CMUT cell, holes are opened in a membrane, and the shape of the cavity portion is controlled by means of the arrangement of the holes.
Also, US Patent Application Publication No. US 2004/0085858 A1 (Patent Document 2) discloses the CMUT cell having a structure in which a cavity is formed by bonding a silicon substrate onto an insulator having a concave portion formed therein.
Further, U.S. Pat. No. 5,982,709 (Patent Document 3) discloses the technology for forming the CMUT cell having a structure in which the size of a cavity portion is defined in advance as a sacrificial layer.